Conventional rotary joints of this kind have a stator fixedly mounted to, for example, the distal end of a robot arm, and a rotor fixedly mounted to a hand side of the robot and rotatably coupled and supported to the stator, and are configured to allow communication of fluid, such as air, cooling water or welding gas, and electricity for power, signals or other purposes between the stator and rotor so that communication of such fluid and electricity can be established even when the hand side of the robot rotates with respect to the stationary side thereof.
Furthermore, for fluid communication, the above rotary joints employ a swivel joint in which plural kinds of fluid passages are arranged between the slide surfaces of the stator and rotor. For power electricity communication, the above rotary joints employ a slip ring consisting of a collector ring mounted to one of the stator and rotor and a slide contact mounted to the other for coming into sliding contact with the collector ring to feed electricity.
There are also known electric signal slip rings for passing electric signals between a stator and a rotor. Proposed as an example of such electric signal slip rings is, as disclosed in Japanese Unexamined Patent Publication No. 10-223346, an electric signal slip ring which includes a stationary side electrode of ring plate shape disposed on the stator side, a rotating side electrode of ring plate shape disposed on the rotor side, and a rolling element disposed between both the electrodes, and which is configured to roll the rolling element on both the electrodes by relative rotation of the rotor to the stator to pass electric signals between both the electrodes.
The proposed electric signal slip ring, however, has a problem that when a plurality of stationary side electrodes and a plurality of rotating side electrodes are arranged to pass different electric signals between the stator and rotor, some or other of the electrodes may produce sliding friction to make it difficult to stably transmit electric signals between the stator and rotor.
Furthermore, electric signal transmission through rolling of the rolling element on the electrodes, though making the occurrence of a slip of the rolling element difficult, cannot avoid the production of wear particles. When the wear particles accumulate on the conductive surfaces after long-time use, it again becomes difficult to stably transmit electric signals.
In addition, in order to ensure stable transmission of electric signals through rolling of the rolling element, it is desirable to reduce the sliding resistance of the rolling element as much as possible.
The present invention has been made in view of the foregoing points, and therefore its object is to improve the structure of the electric signal slip ring in the rotary joint as described above, thereby enabling, even in passing different electric signals between the stator and rotor, stable transmission of the electric signals.